Categories Metabox Enhanced

Try it live in your browser — launch the WordPress Playground demo. Boots a WordPress instance with this plugin active, the Category taxonomy pre-configured as radio buttons, and seeded posts you can edit in the Block Editor or run through Quick Edit / Bulk Edit immediately.

Thanks to Taxonomy_Single_Term, a library created by WebDevStudios, it made my work much easier on creating this plugin.

With Categories Metabox Enhanced, you can:

• Change the built-in Categories metabox/panel to a single term UI, which means replacing the checkboxes with radio buttons or a select drop-down.
• Apply the single term UI to other hierarchical taxonomies from the plugin’s Settings page.
• Configure each hierarchical taxonomy independently — the same install can keep one taxonomy as checkboxes while another is forced to radio.
• Customize the single term UI by setting these options:
  • Option type (checkbox, radio, or select).
  • Priority and position (context) of the metabox placement. Classic editor only.
  • Title of the metabox / Block Editor panel.
  • Whether child-terms should be indented.
  • Whether adding new terms from the metabox is enabled.
  • Whether a term selection is required (Force selection). When on, the classic metabox hides the “None” option, the Block Editor sidebar drops the “— Select —” entry once a term is chosen, and an empty save is substituted with a default term server-side so REST and programmatic callers can’t bypass it.

The single-term invariant is enforced everywhere posts can be saved: the Classic Editor metabox, the Block Editor sidebar panel, Quick Edit, Bulk Edit, the REST API, WP-CLI, and any code that calls wp_set_object_terms() directly. The classic-editor and Block Editor surfaces enforce in the UI; everything else is enforced on the server via the set_object_terms action, which coerces multi-term submissions to the last term and substitutes the default term on empty submissions when Force selection is on.

The substituted default term resolves in this order: the default_<taxonomy> option (e.g. default_category) → the default_term_<taxonomy> option populated by register_taxonomy() default_term arg (WP 5.5+) → the first term ordered by name. Override per taxonomy with the of_cme_force_selection_default_term filter.

The Block Editor sidebar panel is rendered for taxonomies registered with show_in_rest = true. Hierarchical taxonomies that opt out of REST fall back to the Classic Editor metabox path.

1. Upload the category-metabox-enhanced directory to the /wp-content/plugins/ directory
2. Activate the plugin through the ‘Plugins’ menu in WordPress

1. 

   The checkboxes in Categories metabox are replaced with radio buttons

2. 

   The checkboxes in Categories metabox are replaced with a select drop-down

3. 

   A settings page for this plugin

0.9.1

• Fix server-side single-term enforcement, which was registered against pre_set_object_terms — a hook that does not exist in WordPress core — and never executed in 0.9.0. The handler now hooks set_object_terms and re-issues wp_set_object_terms with a corrected list when the single-term contract is violated, so REST and programmatic callers are now actually enforced. The classic-editor metabox path was unaffected; it relies on UI-level enforcement.
• Register script translations for the Block Editor sidebar panel so its strings can be loaded from language packs.

0.9.0

• Add a “Force selection” per-taxonomy setting (defaults on) replacing the previously hardcoded classic-editor behavior. When on, the Block Editor sidebar suppresses “— Select —” once a term is chosen, and the server-side handler substitutes a default term for empty submissions on radio/select taxonomies — closing the bypass that REST and programmatic callers had on the single-term invariant. (The 0.9.0 release wired this handler against the wrong hook name; see 0.9.1.)
• The substituted term resolves in this order: default_<taxonomy> option → default_term registered with the taxonomy (WP 5.5+) → first term by name asc. Filterable via of_cme_force_selection_default_term. Mirrors the classic library’s process_default() pattern.

0.8.0

• Replace the legacy Block Editor integration with a native sidebar panel built on @wordpress/components (TreeSelect / radio tree) and PluginDocumentSettingPanel.
• Add server-side single-term enforcement so REST and programmatic saves can’t bypass the radio/select invariant. (Released against the wrong hook name; superseded by 0.9.1.)
• The classic-editor metabox path is unchanged; it now skips post types that use the Block Editor to avoid duplicate UI.

0.7.1

• Update the Taxonomy_Single_Term library and some cosmetic fixes.

0.7.0

• Support the single term UI in Gutenberg sidebar Categories/Taxonomies panels.

0.6.1

• Remove git submodule from this plugin.

0.6.0

• Indent the child-terms in select options.
• Remove unused CSS and JS files.

0.5.0

• The first version.